INFLUENCE OF TIN POWDER CHARACTERISTICS ON THE SINTERING OF IRON-TIN-COPPER POWDER COMPACTS

Abstract

In view of increasing industrial interest in the use of tin additions as an aid to the sintering of iron-based powder compacts, an examination has been made of the influence of the characteristics of the tin powder on sintering performance.

The effect of additions of narrow size-range fractions of atomized tin powder on the dimensional changes and tensile properties obtained on sintering Fe-Sn-Cu compacts made with –100 mesh (–152 μm) or – 300 mesh (– 53 μm) sponge iron and – 300 mesh (– 53 μm) atomized copper powders has been determined. The compacts contained tin and copper in the ratio 2:3. The narrow size fractions were separated from – 300 mesh tin powder by air elutriation. It was found that the use of coarse tin powder reduced the tensile strength of – 300 mesh iron-based Fe–1% Sn–1 ½% Cu compacts, but had no influence when this mixture was based on –100 mesh iron powder, or when the mixture composition was Fe–2% Sn–3% Cu. The effects have been examined in relation to the sintering mechanism by scanning electron microscopy and by X-ray microanalysis.     

Transient liquid phase sintering of high density Fe3Al using Fe and Fe2Al5/FeAl2 powders Part 1: Experimentation and results

Abstract

High density Fe₃Al was produced through transient liquid phase sintering, using rapid heating rates of greater than 150 K min^-1 and a mixture of prealloyed and elemental powders. Prealloyed Fe₂Al₅/FeAl₂ (50Fe/50Al, wt-%) powder was added to elemental iron powder in a ratio appropriate for producing an overall Fe₃Al (13·87 wt-%) ratio. The heating rate, sintering time, sintering temperature, green density and powder particle size were controlled during the study. Heating rate, sintering time and powder particle size had the most significant influence upon the sintered density of the compacts. The highest sintered density of 6·12 Mg m^-3 (92% of the theoretical density for Fe3Al) was achieved after 15 minutes of sintering at 1350°C, using a 250 K min^{- 1} heating rate, 1-6 μm Fe powders and 5·66 μm alloy powders.

SEM microscopy suggests that agglomerated Fe₂Al₅/ FeAl₂ particles, which form a liquid during sintering, are responsible for a significant portion of the remaining porosity in high sintered density compacts, creating stable pores, larger than 100 μm diameter, after melting. High density was achieved by minimising the Kirkendall porosity formed during heating by unbalanced diffusion and solubility between the iron and Fe₂Al₅/FeAl₂ components. The lower diffusion rate of aluminium in the prealloyed powder into the iron compared with elemental aluminium in iron, coupled with a fast heating rate, is expected to permit minimal iron-aluminium interdiffusion during heating so that when a liquid forms the aluminium dissolves in the iron to promote solidification at a lower aluminium content. This leads to a further reduction in porosity.     

The effect of tungsten particle size on the processing and properties of infiltrated W-Cu compacts

Three tungsten powders with average particle sizes of 8.7, 23.2, and 65.2 μm were used to make W-15Cu compacts. The compacting pressure and sintering temperature were adjusted for each powder to attain the desired skeleton density. Sintered skeletons were then infiltrated with oxygen-free copper at 1200 °C in hydrogen and in vacuum. Results showed that as the tungsten particle size decreased, higher compacting pressures and sintering temperatures were required for the same desired skeleton density. The processing parameters and the tungsten particle size caused variations in the amount of closed pores and the W-W contiguity, which in turn resulted in different infiltrated densities and resistivities. Direct infiltration on green compacts was also examined, and higher infiltration densities and lower electrical resistivities were obtained compared to those obtained by infiltrating sintered compacts. These results are discussed based on infiltrated density, differences in microstructure, and the W-W contiguity.     

News & Views

Abstract

Although hardmetal has been manufactured, by the powder metallurgy route for well over 55 years, the characterization of the starting powders, basically tungsten carbide and cobalt, still commands the attention of powder metallurgists. Three fine cobalt powders and two 1·2 μm (FSSS) tungsten carbide powders, freely available, have been both chemically and physically characterized. Various techniques of measuring physical characteristics, such as FSSS, BET gas adsorption, pore volume and area, sedimentation, SEM powder, and SEM polished and etched sections of copper impregnated powders, have been studied. The techniques have been discussed in relation to their advantages and disadvantages. The measurement by FSSS and BET gas adsorption give fairly reproducible results, as does the sedimentation technique. However, by reference to the two SEM techniques it has been shown that the very important characteristic of particle shape and, more importantly for tungsten carbide powder, particle agglomeration (due to the manufacturing route) is not fully revealed. The two SEM techniques are indispensable for defining shape and agglomeration characteristics. It is suggested that by using the SEM copper impregnated powder method and a semi-automatic image analysing system the true crystal (grain) size distribution of tungsten carbide powders can be evaluated. The chemical purity of cobalt and tungsten carbide powders has significantly increased in recent years. Some preliminary results from milled tungsten carbide powders are discussed. PM/0393     

STUDIES OF THE SINTERING AND HOMOGENIZATION OF NICKEL-COPPER COMPACTS

Abstract

Studies are described of the progress of sintering and alloying in compacts of similar compositions made from nickel-coated copper, copper-coated nickel, and mixed nickel and copper powders. Density losses observed in the early stages of sintering were lower in magnitude and were more quickly recovered in the case of the composite powder compacts. Alloying by diffusion at both 1900 and 2200°F (1040 and 1205°C) progressed most rapidly in compacts prepared from nickel-coated copper powders, and the probable reasons for this observation are discussed in detail. Electrical resistivity was used to follow homogenization of the compacts, and samples were rendered nearly 100% dense by cold working and annealing before making resistivity measurements. Resistivity / sintering-time curves for dense specimens showed no maxima of the type reported by earlier investigators for porous compacts, which were attributed to alloying effects.     

Effects of wet and dry milling conditions on properties of mechanically alloyed and sintered W–C and W–B4C–C composites

Abstract

Tungsten based W–1C and W–2B₄C–1C (wt-%) powders synthesised by mechanical alloying (MA) for milling durations of 10, 20 and 30 h, in wet (ethanol) and dry conditions, were characterised. X-ray fluorescence spectroscopy investigations revealed Co contamination which increased with increasing milling time during wet milling. X-ray diffraction investigations revealed the presence of W and WC phases in all powders, Co₃C intermetallic in the wet milled W–1C powders and W₂B intermetallic phase in both wet and dry milled W–2B₄C–1C powders. As blended and MA processed powders were consolidated into green compacts by uniaxial cold pressing at 500 MPa and solid phase sintered at 1680°C under hydrogen and argon atmospheres for 1 h. X-ray diffraction investigations revealed the presence of W₂C intermetallic phase in sintered composites produced from both wet and dry milled W–1C powders and the W₂B intermetallic phase in sintered material from the wet milled W–2B₄C–1C powder. Sintered composites from wet milled powders showed relative densities >91%, with the maximum density of 99·5% measured for the sintered 30 h wet milled W–2B₄C–1C composites. Microhardness values for the wet milled W–1C and W–2B₄C–1C composites were 2–2·5 times higher than those for dry milled composite powders. A maximum hardness value of 23·7±2·1 GPa was measured for the sintered W–2B₄C–1C composite wet milled for 20 h.     

Processing and properties of sinter hardened Fe–Cr–Mo steels with admixed extra-fine nickel

Abstract

The processing and properties of chromium–molybdenum, powder metallurgy steels with admixed extra-fine nickel (XF Ni) were investigated. Prealloyed Fe–1·5Cr–0·2Mo powder was blended with different quantities of XF Ni, while a hybrid steel with lower Cr content was prepared by blending Fe–1·5Cr–0·2Mo and Fe–0·5Mo prealloyed powders, with additions of XF Ni and copper powders. These steels were compacted into different part shapes in order to evaluate the effect of part thickness on sinterhardening behaviour. These parts were also subjected to different cooling rates after sintering. This study showed that additions of XF Ni improve the compressibility, densification behaviour and mechanical properties of Cr–Mo steels. Furthermore, the properties of the hybrid steel were shown to be either equal to or greater than those of the reference material. Hardenability of all steels was sufficiently high such that part thickness was seen to have negligible impact. Higher cooling rates generally resulted in improved mechanical properties.     

OBSERVATIONS ON THE SINTERING OF COMPACTSFROM A MIXTURE OF IRON AND COPPER POWDERS

Abstract

Three grades of iron powder-an atomized steel powder, a sponge iron powder reduced from magnetite with carbon, and a powder reduced from mill scale with hydrogen were mixed with 3% of copper powder and pressed into compacts. The diametral dimensional changes of the compacts during sintering below and above the melting point of copper were measured, their microstructures examined, and both related to the characteristics of the powders, particularly their specific surface. During sintering below the melting point of copper, compacts of all three powders shrank. Micrographic examination showed that the copper is transported by solid-state diffusion along the surfacesand grain boundaries of the iron powder particles. During sintering above the melting point of copper, compacts of the atomized and the MH-100 sponge iron powders grew while those of the hydrogen reduced mill-scale powder shrank. This phenomenon is related to the different mode of penetration of liquid copper in the compacts from the three powders, observed in the microstructures of the compacts.     

W-35%Cu液相活化烧结工艺研究

本文从钨铜两相迁移机制和固溶扩散的角度 ,实验研究了钨铜复合粉的活化和常规烧结 ,分析了钨铜复合粉液相活化烧结过程中球磨时间、烧结温度、保温时间和升温速度对材料致密度的影响。结果表明 ,短时间球磨的钨铜复合粉具有良好的烧结性能 ,烧结温度和保温时间对材料致密度影响很大 ,活化烧结能有效降低烧结温度 ,提高烧结致密度     

Sintering of Tungsten and Tungsten Heavy Alloys of W–Ni–Fe and W–Ni–Cu: A Review

Tungsten is a refractory metal possessing good mechanical properties of high strength, high yield point, and high resistance to creep. Therefore, tungsten and its alloys are used in many high temperature applications. Due to the high melting point, they are generally processed through powder metallurgy method. The powders are compacted using die pressing or isostatic pressing. The compacts are sintered in a sintering furnace to achieve high density, thereby, making the metal suitable for further processing. This article reviews the recent research findings of consolidating tungsten and its alloys (W–Ni–Fe and W–Ni–Cu), from preparation of powder alloys to sintering of the compact. The advances in sintering are based on the objective of achieving good densification of the metal at lower temperature and at faster rate. The use of microwave sintering and spark plasma sintering techniques resulted in significant reduction in sintering time and producing products of good mechanical properties.     

Effects of nickel addition on properties of Ag–W electrical contact materials

Abstract

Tungsten and silver powders doped with small amounts of nickel were milled in a heavy duty attritor for 150 min to constitute the W–35 wt-%Ag composition. Milled powders were compacted, sintered, furnace cooled, and re-sintered using industrial conditions. Scanning electron microscopy (SEM) investigations revealed homogeneous dispersions of silver and tungsten regions both in the as milled powders and the as milled compacts. The X-ray results taken from the milled powders and the as milled compacts showed the presence of the characteristic W and Ag peaks. Bulk hardness values of the sintered compacts increased with increases in nickel additions. The arc erosion characteristics of the W–35 wt-%Ag com pacts indicated that the erosion rate of the contact material declined with nickel additions. Contact life expectancy increases about 10 fold for the W–35 wt-%Ag compact containing 1 wt-%Ni from the one with no Ni content.     

Improving Conductivity of Copper PM Parts by Pretreatment of Green Compact

Abstract

There is a maximum green density to which pure copper can be compacted and still exhibit no growth upon sintering. This limits the maximum density and thereby the maximum conductivity and strength that can be achieved with sintered copper bodies. The swelling of a part pressed above this critical compacting pressure is caused by premature closure of the outer pores of the compact, thus sealing off the egress of internally generated gases. In this study a number of carefully selected chemical compounds were added to the as-pressed compacts using two methods in an effort to find a way of keeping these outer pores temporarily open until all internal gases are eliminated. These pores are then sealed off in the later stages of sintering, resulting in a higher density body with improved electrical conductivity throughout. Two compounds proved to be most effective in accomplishing this without leaving a residue deleterious to conductivity. Two commercial copper powders were die pressed into thick discs and then impregnated with an aqueous salt solution, the preferred treatment. After a special sintering procedure, the best conductivity obtained in an 18 mm diameter × 10 mm thick disc using the most responsive powder and treatment was 93·9% IACS at the centre and 94·2% IACS at the surface. Untreated, the same powder showed a maximum centre conductivity of 89% IACS.     

FEASIBILITY OF PRODUCING DISPERSION-STRENGTHENED CHROMIUM BY BALL-MILLING IN HYDROGEN HALIDES

Abstract

Chromium, with and without 4 vol.-% thoria, and nickel powders were ground to fine powder sizes by ball-milling in gaseous hydrogen halides. After reducing the milled chromium in flowing hydrogen under pulsating pressure at ～680°C, submicron-size powders with 4–500 ppm residual halogens were obtained. The compacted chromium–thoria alloys had interparticle spacings ranging from 2·1 to 6·5 μm. After 100 h at 1318°C the interparticle spacing of the 2·1-μm alloy increased to 5·2 μm. Submicron-size chromium and nickel powders were also obtained by pulsating hydrogen reduction of their chlorides.     

Effects of sintering process and heat treatments on microstructures and mechanical properties of VANADIS 4 tool steel added with TiC powders

Abstract

In this study, commercial VANADIS 4 (V-4) tool steel powders were classified by sifting, which was previously the matrix, and fine TiC powder was used as an additive to produce a new material with high hardness and wear resistance, via powder metallurgy and a sintering process. Experimental results showed that the transverse rupture strength of the original V-4 steel powder was 678·5 MPa and was enhanced to 868·6 MPa below 25 μm, after the addition of 35 wt-%TiC powders through sintering at 1400°C. In addition, the hardness increased to 86·2 HRA, transverse rupture strength reached 1059·3 MPa and porosity decreased to 1·2% of the V-4 steel powders (below 25 μm) added with 35 wt-%TiC after sintered at 1400°C and annealed at 850°C, followed by quenching at 1030°C and tempering at 200°C.     

The effect of Mo addition on the liquid-phase sintering of W heavy alloy

The morphological and compositional changes of grains have been investigated in the initial stage of liquid-phase sintering of W-Mo-Ni-Fe powder compacts. Both large (5.4-μm) and small (1.3-μm) W powders have been used to vary their time of dissolution in the liquid matrix. When 8OW-10M0-7Ni-3Fe (wt pct) compacts of fine (about 1- to 2-μm) Mo, Ni, and Fe and coarse (5.4-μm) W powders are liquid-phase sintered at 1500 °C, the Mo powder and a fraction of the W powder rapidly dissolve in the Ni-Fe liquid matrix. The W-Mo grains (containing small amounts of Ni and Fe) nucleate in the matrix and grow while the W particles slowly dissolve. In this transient initial stage of the liquid-phase sintering, duplex structures of coarse W-Mo grains and fine W particles are obtained. As the W particles dissolve in the liquid matrix during the sintering, the W content in the precipitated solid phase also increases. The dissolution of the small W particles is assessed to be driven partially by the coherency strain produced by Mo diffusion at the surface. During sintering, the W particles continuously dissolve while the W-Mo grains grow. When the compacts are prepared from a fine (1.3-μm) W powder, the W grains dissolve more rapidly, in about 1 hour, and only W-Mo grains remain. These observations show that the morphological evolution of grains during liquid-phase sintering can be strongly influenced by the chemical equilibrium process. formerly Graduate Student, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology     

Sintered copper–graphite powder compacts forindustrial applications

Abstract

Studies were made on copper/graphite based powders and sintered compacts for industrial applications. The dependence of particle shape on friction in the powder mass, compression ratio, and electrical receptivity of powder metallurgy components was studied using near spherical precipitated copper powders and angular or flakelike powders generated by mechanical comminution. Results reveal that powders with particles that are nearly spherical in shape have lower friction, lower compression ratios, and higher electrical resistivities in sintered compacts than powders with acicular or flakelike particles. Also, the effects produced by the small additions of lead and zinc (up to 2·5 wt-%) on the electrical resistivity and hardness of sintered copper–graphite compacts are also presented, and the influence of variation of briguetting pressure is discussed.     

机械热化学法制备的Mo-Cu复合粉末及其性能

钼酸铵热解氧化物与铜粉经过球磨混合后,在H2气氛下进行共还原,制得Mo-30Cu复合粉末,利用X射线衍射、SEM等测试分析手段对复合粉末进行表征,研究粉末的压制行为和烧结性能,并研究烧结温度对Mo-Cu合金的致密度、热导率和电导率的影响。结果表明:采用机械-热化学法可以制备出颗粒均匀的Mo-Cu复合粉末,该粉末具有良好的压制性;随烧结温度的升高,Mo-Cu合金的致密度、热导率和电导率提高,经1 280℃烧结后,合金的致密度可达99%以上,显微组织分布均匀,合金的热导率最高达到196.5(W.m-1.K-1),电导率达50.5 IACS。     

Preparation of tungsten base composite powder by electroless nickel plating

Abstract

Fine tungsten powder with an average size of 8 μm was coated by electroless nickel plating with hydrazine and sodium hypophosphite reducing agents to obtain Ni and Ni–P coatings, respectively. The influence of process parameters such as temperature, pH and time of electroless plating was investigated. As coated composite powders were characterised by energy dispersive spectrometer analysis and scanning electron microscopy. It was found that, high homogeneity Ni/Ni–P coatings are deposited around the tungsten particles. Also it was shown that deposited mass on the powders increases as the temperature and pH of bath increase, but with different deposition rates depending on coating type. Furthermore, other results indicate that at higher pH values, the P content in the Ni–P coating decreases, leading less impurity in the final composite powders.     

Spark plasma sintering of cryomilled copper powder

Abstract

The densification and sintering behaviour of a cryomilled copper powder (grain size of 17±2 nm and dislocation density of 6·26±0·04×10¹⁶ m^?2) were investigated and compared to those of an atomised copper powder with the same mean particle size in order to highlight the effect of the nanostructure on spark plasma sintering (SPS). Oxygen and nitrogen contamination of the cryomilled powder gives rise to extensive degassing during SPS up to 400°C. The cryomilled powder is more resistant to plastic deformation than the atomised one, but the huge density of dislocations and grain boundary activates sintering at low temperature. Densification is therefore promoted by deformation in the atomised powder and by sintering shrinkage in the cryomilled one. As a consequence, in the SPS conditions investigated, the atomised specimen is densified but not sintered, while the cryomilled one is effectively sintered and consequently densified.     

EFFECTS OF PARTICLE SIZE ON THE SINTERING KINETICS IN TUNGSTEN POWDER

Abstract

An experimental study has been made of the effects of initial particle size on sintering kinetics in tungsten powder within the temperature range 1100–1500°C. Particle size, compacting pressure, sintering time and temperature all influence the rate of sintering. Isothermal changes in density and volume have been measured. The results indicate grain-boundary diffusion as the mechanism principally responsible for material transport in the case of particle sizes <4 μ Surface diffusion appears to bethe mechanism of material transport in compacts with particle sizes of 14– 16 μ The temperature-dependence of the rate of sintering is characterized by activation energies of 101± 2 and 72± 2 kcal/mole for fine particles (< 4 μ) and coarse particles (14–16 μ), respectively.